1. Field of the Invention
The present invention relates to the field of fabrication of integrated circuits, and, more particularly, to the field of so-called advanced process control (APC).
2. Description of the Related Art
Fabrication of integrated circuits requires the precise formation of very small features with a very small tolerance for error. Such features may be formed in a material layer formed above an appropriate substrate, such as a silicon substrate. These features of precisely controlled size are generated by patterning the material layer by performing known photolithography and etching processes, wherein a masking layer is formed over the material layer to be etched to define these features. Generally, a masking layer may consist of or is formed by means of a layer of photoresist that is patterned by a lithographic process. During the lithographic process, the photoresist may be spin-coated onto the wafer surface and is then selectively exposed to ultraviolet radiation. After developing the photoresist, depending on the type of resist, i.e., positive resist or negative resist, the exposed portions or the non-exposed portions are removed to form the required pattern in the layer of photoresist.
Since the dimensions of the patterns in sophisticated integrated circuits are steadily decreasing, the equipment used for patterning device features have to meet very stringent requirements with regard to resolution and overlay accuracy of the involved fabrication processes. In this respect, resolution is considered as a measure of specifying the consistent ability to print images of a minimum size under conditions of predefined manufacturing variations. One important factor in improving the resolution is represented by the lithographic process, in which patterns contained in a photo mask or reticle are optically transferred to the layer of photoresist via an optical imaging system. Therefore, great efforts are made to steadily improve optical properties of the lithographic system and of the material layer to be patterned.
The relevant properties of the lithographic system may be numerical aperture, depth of focus and wavelength of the light source used. The relevant properties of the material layer to be patterned may, for example, be influenced by the type of photoresist used, baking temperature and thickness variations of resist layer, reflectivity of underlying material layer and planarity of underlying layers. The quality of the lithographic imagery is extremely important in creating very small feature sizes.
Of at least comparable importance, however, is the accuracy with which an image can be positioned on the surface of the substrate. Integrated circuits are typically fabricated by sequentially patterning material layers, wherein features on successive material layers bear a spatial relationship to one another. Each pattern formed in a subsequent material layer has to be aligned to a corresponding pattern formed in the previously patterned material layer within specified registration tolerances. This is a particular challenging task due to the typically performed stepper exposure process so that for each exposure field, for example, a single die is exposed separately and the entire substrate is, consequently, exposed step by step in a sequential manner. In addition, not only the exposure process may be affected by non-uniformity problems, but, for example, deposition, etch, implantation and annealing processes may also be concerned.
As the minimum device dimensions, also referred to as critical dimensions (CD), steadily decrease, it is therefore desirable to minimize feature variations not only from wafer to wafer but also across the entire wafer surface to allow semiconductor manufacturers to use processes the tolerances of which may be set more tightly to achieve improved production yield while at the same time enhance device performance in view of, for example, operational speed. Otherwise, the fluctuations across the wafer and the wafer-to-wafer variations may be taken into account, thereby requiring a circuit design that tolerates higher process discrepancies.
In principle, there are two different strategies to overcome the issues caused by the non-uniformities across the wafer and the wafer-to-wafer variations. The non-uniformities may be reduced, for example, by an improved process control or may be compensated for by accordingly modifying a process that may have a compensating effect. An example for compensation of across-wafer non-uniformities is a position-dependent adapted exposure parameter in the gate exposure process to compensate for any position-dependent variations causing variations in the electrical parameters of field effect transistors to be formed. The gate exposure process is typically employed for compensation of non-uniformities across the wafer, as the gate exposure process provides both an effective compensation mechanism and a position-dependent control feasibility, due to the usage of the stepper technique. The position-dependent control may be carried out, for example, by an accordingly adapted so-called exposure map defining exposure parameters for each single exposure step.
On the other hand, for example, gate length variations as patterned may be compensated for position independently, wafer by wafer or lot by lot by adapting the width of subsequently formed gate sidewall spacers accordingly. An example, which does not concern lithography processes, is the compensation of across-wafer non-uniformities during the metallization layer formation, such as the compensation for a non-uniformity of a removal rate of a chemical mechanical polishing (CMP) process by an accordingly adapted plating process forming a corresponding compensating deposition profile.
As mentioned above, an improved process control enables, compared to a compensation technique, a straighter reduction of wafer-to-wafer and across-wafer non-uniformities. Recently, a process control strategy has been introduced and is continuously improved, allowing a high degree of process control, desirably on a run-to-run basis, without the necessity of an immediate response of a measurement tool. In this control strategy, so-called advanced process control, a model of a process or of a group of interrelated processes is established and implemented in an appropriately configured process controller. The process controller also receives information related to the type of process or processes, the product, the process tool or process tools in which the products are to be processed, the process recipe to be used, i.e., a set of required sub-steps for the process or processes under consideration (containing possibly fixed process parameters and variable process parameters), measurement results of previously processed products or test substrates, and the like. From this information, which may also be referred to as history information, and the process model, the process controller determines a controller state or process state that describes the effect of the process or processes under consideration on the specific product.
With reference to FIGS. 1a and 1b, an illustrative example of an advanced process control (APC) will now be described. FIG. 1a schematically shows an APC architecture that is exemplified for a photolithography process. A photolithography station 100 comprises a photoresist application module 150, a stepper exposure and development module 140, a develop inspect critical dimension (DICD) module 120 and a rework module 160. An advanced process controller 110 is operatively connected to the photolithography station 100. Moreover, the process controller 110 is configured to receive information from the photo-resist application module 150 and from the DICD module 120 of the photolithography station 100. Furthermore, the process controller 110 may receive information related to a product substrate 130 to be processed by the photolithography station 100 and from the product substrate 130 at a later process step, for example, from a metrology tool and/or an electrical test station 170, and information related to a critical dimension (CD) target value T(x,y) 112 that may be provided position dependently.
In operation, a photoresist layer is formed on the product substrate 130 in the photo-resist application module 150 according to a conventional technique, typically, by a spin-on technique. In the module 140, the substrate 130 is exposed by well known exposure techniques, typically by a deep ultra-violet light source, and the exposed resist layer is developed. Subsequently, the developed resist layer is subjected to a DICD measurement to assess the quality of the exposure process. In case the DICD tolerances do not meet the requirements, the substrate may be subjected to a rework process, wherein the developed resist is removed from the substrate to repeat the photolithography process with accordingly adapted parameters. The rework of substrates removing a photoresist layer does, in general, not unduly affect the substrate so that the photolithography process may be repeated several times in case of exposure failures. The reflectivity of amorphous carbon, which may be employed in metallization layers of modern semiconductor devices due to their low permittivity, may, however, be modified during the rework process. Thus, the photolithography of reworked substrates comprising amorphous carbon layers may be affected.
The advanced process control of the system shown in FIG. 1a will be explained with reference to the flowchart shown in FIG. 1b. In step 105, the process controller 110 is initialized, i.e., the process controller 110 is set to an initial process state. A process state in this example may represent, for instance, the state of the exposure light source. The process state may also represent the reflectivity of the substrate surface and/or the type and thickness of the photoresist. Since, generally, the amount of history information available upon initialization of the process controller 110 may not be sufficient to determine a process state, the initial state is set in advance and selected so that the effect of the tool is expected to be within the process specifications. The product substrate 130 is then processed with process parameters adjusted on the basis of the initial process state.
In step 115, the process controller 110 determines a process state on the basis of the process model implemented and the history information received from, for example, the metrology system 170, the photolithography station 100, a further product substrate 130 to be processed and the corresponding CD target values. It should be noted that, in particular, the measurement results obtained from the metrology tool 170 may be delayed or may even not be available unless a plurality of product substrates 130 is completely processed. Thus, the process controller 110 establishes the currently valid process state on the basis of the available information and the process model to “predict” the effect of the photolithography process on the product to be processed and to adjust process parameters correspondingly to achieve the predicted effect. For example, the process controller 110 may estimate the maximum intensity of the exposure light source from the available information, such as the number of products that has already been processed, type of process to be performed and the like, and estimate the “state” of the process and correspondingly adjust a process parameter, for example the exposure time, to obtain the specified process result. In other processes, the process state may represent the removal rate of a CMP tool, the etch rate in an etch tool, the deposition rate in a deposition tool, and so on.
As indicated in step 125, the determination of the process state may require the processing of one or more pilot substrates to improve control quality, since the accuracy of the determined process state may significantly depend on the available history information, the amount and the accuracy of which increases with an increasing number of processed products.
In step 135, the process state is updated, that is, a new or advanced process state is determined on the basis of the previous process states, including the previously obtained history information. Preferably, the advanced process state is established on a run-to-run basis, that is, prior to processing an individual product 130, the corresponding process state is established, and on the basis of the currently valid process state, the process parameter(s) may accordingly be adjusted.
As indicated in step 145, the process flow continuously updates the process state when no reset event occurs. Generally, process control quality improves as the amount of history information increases, unless the history information indicates that predefined specifications are no longer met. For instance, the lifetime of the exposure light source has expired or will soon expire, reworked substrates, in particular when covered with amorphous carbon layers, are processed, the type of photoresist is to be changed, the type of product is to be changed, the CD target value has to be changed and the like. Any of these events may render the process state unpredictable and, therefore, the process controller 110 is re-initialized with the initial state set in advance, and the process continues as depicted in FIG. 1b on the basis of newly gathered history information after the reset event. Even if these events do not require a re-initialization of the controller, the adaptation to the resultant new controller state may affect the process quality in the transition phase.
It should be noted that the system shown and described with reference to FIG. 1a is only an illustrative example, wherein the process controller 110 is connected to a single process station. However, the process controller 110 may be configured to perform several control operations with a plurality of different product types and process recipes, as well as with more than one process system.
Although the advanced process control, as exemplarily described above, provides significant advantages compared with process controls based on, for example, measurement mean values, the occurrence of abrupt events conventionally requires a re-adaptation lasting a period that may concern several substrates, or a re-initializing (reset events) of the process controller, resulting in a reduced process quality in an early state after the re-initialization and also possibly requiring the processing of additional pilot substrates.
Any adaptation to abrupt events and re-initialization, however, reduces the process capability due to a wider range of tolerances of the process during the period of adaptation or after the re-initialization, and entails a reduced throughput due to the processing of pilot wafers and a reduced yield caused by a higher probability of device failures.
In view of the above-identified problems, a need exists to provide an improved advanced process control strategy, in particular for a lithography process, wherein one or more of the above constraints may be avoided or at least reduced.